X-ray security inspection systems for cargo and shipping containers typically use transmission radiographic techniques. A high inspection throughput of cargo and cargo-carrying vehicles is at a premium. Consequently, it is desirable that an entire plane through the cargo be probed simultaneously, and one inspection modality employs a fan-shaped beam to produce images of a target object while the fan beam and detectors are moved relative to the object. Alternatively, the object may be moved in a direction substantially perpendicular to the plane of the fan beam. In cases where illumination is provided by a fan beam of x-ray radiation, useful spatial resolution of contents of the inspected object is typically provided by a plurality of detector elements. The spatial pixel resolution is governed by the dimensions of the detector elements in a plane normal to the propagation direction of the beam, or else by post-collimators limiting the field of view of each detector element.
In cargo imaging applications, it may be necessary for the penetrating radiation to penetrate a significant thickness of highly attenuating material, and a requirement for penetration of more than 300 mm of steel equivalent is not unusual. As used herein, a penetration depth quoted in length of steel equivalent refers to the maximum steel thickness behind which a lead block can still be seen. For thicknesses of steel exceeding the penetration capacity of a particular imaging system, the image will be completely dark, and the block will not be seen.
To ensure the required penetration, inspection systems employed for the inspection of cargo, and in certain industrial applications, typically use x-rays with a maximum energy of several MeV, and, more particularly, in current systems, energies up to about 9 MeV. As used herein and in any appended claims, energies in excess of 1 MeV may be referred to as hard x-rays or high-energy x-rays.
Detector modules used in high energy x-ray transmission imaging systems require precise alignment relative to the centerline of the beam plane to optimize the imaging capability of the equipment. This is traditionally done using a combination of precision control surfaces on the base structure and manual adjustors to position the detector element.
As an example of an alignment requirement, consider a system in which a resolution of 15 mm is required of a voxel situated 4 m from the source of radiation. Say the detector elements are located at a distance of 8 m from the source, thereby allowing the imaging of a cargo vehicle having a cross section of 3 m width by 5 m height. Since it is the nature of Bremsstrahlung that x-ray generation at the target is more sharply forward-peaked for higher energies, the beam will become more narrowly peaked as it hardens on propagation through attenuating matter in the inspected object. One might assume that the detector must be centered on the beam to within half the dimension of a resolution element, or, say, 10 mm at 8 m. For scale, a resolution requirement of 10 mm at a distance of 8 m is an angular tolerance of resolution of 1.25 mr (0.07°), or comparable to the width of the eye of a 9/65 sewing needle at a distance of 8 inches from a tailor's eye—a formidable task, when considering that each of an array of nearly 1800 detector elements must be aligned to that accuracy in each of two dimensions in order to provide uniform resolution across the image field.
In ordinary alignment practice, x-ray detection tools, such as small ion chambers, may be used to coarsely measure the beam profile and locate the position near the detector module array. Then a variety of optically based tools (laser modules, surveyor's transit, digital levels, etc.) are employed to assist the technician to position the modules to the expected beam location. Due to the nature of ionizing radiation, the technician performing the alignment should not be near the equipment while the source is energized. Therefore, the manual adjustments are never made with the beam on, and no direct feedback of how well the detector is aligned while the changes are being made is available. Moreover, the process is made substantially more arduous if the geometry of the system is anything other than rectilinear.
Since there are many sources of error in each of the measurements taken and adjustments made, the alignment process is in essence a “trial and error” based iterative method. The time consuming nature of the process results in a labor intensive effort that can be quite costly and ultimately the detectors may not end up in an optimal position. Since the use of optical alignment tools requires line of sight from the various components in order to function, the alignment process can require the removal of covers to gain access to the detectors and adjustors and may expose the detectors and other sensitive components within the equipment to potentially damaging environmental factors.
X-ray detector modules may also incorporate a variety of post collimators mounted directly in front of the detector elements of the detector modules in order to reject in-plane and out of plane noise sources, such as scattered radiation, from reaching the detector elements. The incorporation of these technologies makes the detector modules more sensitive to misalignment than traditional detector modules, and, if the geometry of the system is non-rectilinear, the alignment is all the more difficult.
For practical and effective deployment, an x-ray inspection system must provide for alignment in the field at the time of installation as well as for ready realignment to minimize down time. As systems are required to provide increasingly higher resolution and larger scan areas, the number of detector elements increases, and the sensitivity to misalignment increases as well. Non-rectilinear geometries make the process all the more arduous. All of these factors make the process of alignment much more critical to the ultimate performance of the machine than may be accommodated using existing means that are known in the art.